Process for electrodialyzing liquids



Aug. 19, 1958 N. w. ROSENBERG 2,848,403

PROCESS FOR ELECTRODIALYZING LIQUIDS Filed May 6. 1954 2 Sheets-Sheet 1 Inventor, 3 Norman Wfims'ener Aug. 19, 1958 N. w. ROSENBERG 2,848,403

PROCESS FOR EDECTRODIALYZING LIQUIDS 2 Sheets-Sheet 2 Filed May 6. 1954 Inventor,

- jvarman Wfiosener 2y v United States Patent PROCESS FOR ELECTRODIALYZING LIQUIDS Norman W. Rosenberg, Newton Center, Mass., assignor to Ionics, Incorporated, Cambridge, Mass., a corporation of Massachusetts Application May 6, 1954, Serial No. 428,072

9 Claims. c1. 204-1s0 The present invention relates to improved method employing electrical energy to effect migration of electrolytes from one solution to another across ion selectively permeable membranes.

The apparatus and operation of a membrane demineralizer for liquids is disclosed in the application for patent by W. E. Katz and N. W. Rosenberg Serial No. 307,302, filed August 30, 1952, now Patent No. 2,694,680. Certain difficulties and disadvantages in such units to adapt them to economic use for moderate capacity may be overcome by the present invention. This application is a continuation-in-part of applicant's prior copending case Serial No. 329,639, filed January 5, 1953.

The invention utilizes a membrane demineralizer having a set of diluting chambers (for a first electrolyte-containing solution) alternately disposed between a set of concentrating chambers (for a second such solution) and in addition at least two electrode chambers in which the current enters and leaves the demineralizer. Washing chambers, if so desired to minimize contamination from the products of the electrode chambers, may be interposed adjacent the cathode and anode chambers of the cell and their respective concentrating and diluting chambers and may be hydraulically independent or manifolded to diluting or concentrating streams. The aforementioned chambers are separated by barriers of intrinsically electrolytically-conductive hydraulically-impermeable ionselective permeable membranes through which dissolved electrolyte is transferred from the diluting chambers to the concentrating chambers by means of a direct electric current in series across the membranes and the chambers defined between them. Such apparatus are more fully described in a copending application to W. E. Katz and N. W. Rosenberg bearing Serial No. 300,302 filed July 22, 1952. It will be apparent that such a membrane demineralizer may consist of an extremely expanded arrangement wherein a great number of concentrating and diluting chambers are employed, for example, 100 or more of each, but nevertheless having two end chambers containing the cathode and anode.

I have observed that certain empirical relationships govern the operation of membrane demineralizers. For instance, it has been found that as the current applied to a membrane demineralizer is increased while other operating parameters are held constant, a critical current density is found above which the apparent resistance of the demineralizer increases and pronounced pH changes are found in the solutions treated. It is, therefore, desirable to operate membrane demineralizers at current densities below this critical point and it is found that the critical current is closely directly proportional to the concentration of the solution treated in the diluting chambers and to a power less than proportional to the velocity of solution in the diluting chambers.

It is important in many practical applications of membrane demineralizers to operate at relatively high current densities in order that high output rates may be obtained in small apparatus. It is, therefore, required to operate at high current densities and consequently at high velocity. To a certain extent this may be employed by using spacers having tortuous configuration cut out therefrom forming the treating chambers so that the solutions are caused to course back and forth over the surfaces of the adjacent membranes. This expedient, as described in copending application to Norman W. Rosenberg, Serial No. 299,592, filed July 18, 1952, now Patent No. 2,708,658, increases the linear velocity at which the solutions pass across the membrane at the same volume flow rate. The formation of polarized solution films can be effectively minimized over a wide range of current densities and concentrations, yielding correspondingly wider ranges of maximum current and voltage efiiciencies,

when the solution undergoing treatment is forced to fol-' low a narrowly confined tortuous path while it is in contact with the membranes. For any given spacing between membranes such narrowly confined, tortuous paths cause an increase in the linear velocity of the flowing solution, thereby substantially reducing polarization effects. Furthermore, the spacers can be of different thickness so that at equal pressures the flow rates and velocities are not necessarily the same in both diluting and concentrating streams. It is easily seen that the hydraulic pressure required to cause solution to flow at the required rate in such a spacer is much increased over that required for a spacer having no such tortuosity. There are, therefore, practical limits to the extent of tortuosity permitted before impractical pressure drops through such a spacer are encountered and it will, therefore, be apparent that in many cases the volume flow rate sufiicient to result in a velocity which will permit a relatively high current density is so high that only a small fraction of the electrolyte in the solution can be removed by a single passage through a membrane demineralizer. One way of solving this problem is to arrange several membrane demineralizers in series with respect to the flow of solution in the diluting chambers whereupon booster pumps may be interspersed between the demineralizers to re-pressurize the flowing solution. In this way, it is possible to obtain sufficiently high velocity to permit the use of relatively high current density and, therefore, high output rates while simultaneously achieving the required extent of demineralization and which will accordingly avoid undue hydraulic pressures in any demineralization.

It is found that in the operation of demineralizers having effective spacer tortuosity that relatively high hydraulic pressures are required to achieve the necessary velocity for the passage of the required current, for example, 15 or 20 or more lbs. per square inch. It will be obvious that in the absence of solution pressure through the concentrating chambers of such a demineralizer, the diluting chamber pressure will be applied across the membrane bounding said diluting chamber. It is well known that suitable demineralizers of this type should use as thin membranes and as thin spacers as are practically attainable or operable, in order to minimize ohmic power loss. When a thin membrane, for example, a material of between 0.1 and 1 mm. in thickness, is subjected to even a relatively low hydraulic pressure on one side without being balanced on the other, it will tend to leak or break if fragile or it will bow into the adjacent compartment. In the latter case when this adjacent compartment is also thin, say of the order of 0.5 mm. to 2 mm. a bowing of the membrane of a few tenths of a millimeter will significantly restrict the actual thickness of the adjacent compartment and widen the actual thickness of the pressurized compartment. The flow through the latter (pressurized) compartment, for example, will therefore occur at a lower linear velocity thereby increasing the tendency towards polarization" which limits the rate of throughput. Thus, the existence of such one-sided pressure is frequently deleterious to the operability of the unit as well as the long life of the membranes due to mechanical stress and possibly acid-base attack resulting from polarization. In extreme cases of bowing even electrical short circuiting can result.

It has been found that these disadvantages can be overcome by substantially equalizing the pressure across membranes at any point thereof. I now propose that the spacer forming the concentrating chamber be substantially similar in tortuosity but not necessarily of the same thickness as that of the diluting chamber and that means he provided to flow the concentrating solution in parallel to the diluting solution and at a velocity sufficient so that the hydraulic pressure through the concentrating chamber shall vary in similar fashion to that in the diluting and thus everywhere oppose with an equal pressure that pressure resulting from the flow of solution in the diluting chambers. While my present preferred range of flow pressures in the unit is from 5 to p. s. i., a pressure range of 2 to 50 p. s. i. is also feasible. Substantially similar configurations of spacers in this sense are defined as configurations in which the pressure drop across a membrane at any point thereof can either be substantially equalized or can represent only a small fraction, such as less than 20% of the manifold-to-manifold pressure drop, when the inlet manifold pressures of both (or all streams) are equalized and when the outlet manifold pressures of both (or all) streams are equalized.

This type of flow which is parallel in its passage through the chambers of the apparatus would at first thought appear to be contrary to accepted practices in similar mass transfer processes wherein countercurrent flow is considered as always a more desirable operating method as, for example, in heat exchangers and in dialysis equipment. In general, the advantage of counterfiow operation in chemical engineering lies in the fact that the driving force (concentration gradient, temperature difference) is relatively constant in counterflow systems, whereas it varies widely in parallel flow. Take, for example, the case of the dialysis of a cellulose-containing caustic solution with water.

In countercurrent flow, the influent cellulose solution which is rich in caustic faces of the effiuent water solution also rich in caustic, and the effluent cellulose solution which is lean in caustic faces pure water. Dialysis is thus effective throughout the dialyzer. In parallel concurrent flow, on the other hand, the influent cellulose solution is opposite pure water thus having a high driving force, but the efiiuent cellulose solution, lean in caustic, is opposite the richest caustic Water solution so that almost no dialysis takes place. This principle is 'of importance in electrodialysis with permselective membranes only to the extent that salt can diffuse therethrough under a concentration gradient. As the efficiency of many membranes, for example those shown in U. S. Pat. No. 2,636,851 issued April 28, 1953, becomes high, salt diffusion becomes negligible. In other words, in electrodialysis, the differencein operation between parallel and countercurrent flow was found to be negligible at low production rates i. e. back leakage of salt was not found to be significant when operating in a parallel fashion. However, as the production rate in the two types of operation was increased, serious inter-cell leakage and membrane deformation in the countercurrent '4 HOW case destroyed the effectiveness and severely limited the output of this operatingmethod. On the other hand, parallel flow at equalized pressures permitted usage of high pressures necessary to obtain high linear velocities which in turn permit high current densities, without causing undue mechanical and hydraulic strains on the membranes. Thus, operation of this invention yielded an unexpected result; i. e. that the effects of back diffusion are entirely unimportant in operating the present unit at high current densities and that a far more important consideration is the maintenance of equalized pressures across all membranes to prevent deformation and back leakage.

It should also be noted that it is conventional to arrange many pairs of diluting and concentrating chambers between a single pair of electrodes in order that electrical energy consumed at the electrodes will be a small fraction of that actually consumed in useful work. It has also become apparent that, for example, for membrane demineralizers of moderate capacity or size, viz., of a few gallons per minute, the aboverequirements are not readily achieved in demineralizers described in the above patent application inasmuch as the requirement of several stages leads to the necessity of having only a few pairs of chambers between a single pair of electrodes in each stage. Similarly, the requirements of high velocities leads to the necessity of re-pressurizing streams of small volume between stages and, therefore, to undue complication in de'mineralization apparatus and to increased investment and operating costs. It will also be apparent that an apparatus which was constructed to have a given number of stages may not be easily adapted to another treatment in which a different number of stages is required.

I have now found that a demi'neralizer may be made by utilizing a single expanded membrane demineralizer having many pairs of diluting and concentrating chambers and a single pair of electrodes by causing the solution which is to be diluted to recirculate, preferably from a reservoir, through the diluting chambers of the demineralizer at a velocity sufficiently high to permit the required electrical current density to be applied without polarization, and periodically discharging the contents of the reservoir and diluting chambers automatically when they have obtained the required extent of demineralization. Alterna tively, a continuous product can be obtained by adjusting a bleed from the dilute reservoir at a lower rate than the recirculation rate through the unit. This principleis often particularly advantageous for apparatus of moderate capacity where it is uneconomical to require an operator in constant attendance to charge and discharge the recirculating system, and I have, therefore, proposed a certain combination of sensing and control elements which will automatically govern the apparatus.

Recirculation is 'anideal'method of permitting such high current densities to be utilized in a unit containing deformable membranes. Especially,'it offers a highly versatile method of varying the concentrate and dilute feed rates to theapparatus as a whole over very wide ranges relative to one another without significantly affecting the hydraulic pressure balance within the chambers of the unit.

It will, however, be obvious that it is not feasible in many instances to permit the volume of concentrating solution discarded by the apparatus to be equal to the volume of diluting solution produced, and, in many cases it is desirable to limit the concentrating solution to not more than 25% of the volume of thediluting solution. I, therefore, propose that the concentrating solution be recycled, preferably through a reservoir tank to permit its repeated use in the concentrating chambers. This stream may be periodically or continuously charged and recharged as described above for the diluting solution.

Many naturally occurring waters contain substantial amounts of Mg, Ca, and bicarbonate ions, and it is well known that the addition of alkali will cause the precipitation of insoluble salts of these ions which are usually magnesium hydroxide and calcium carbonate. For the economical operation of membrane demineralizers on natural waters the use of the same natural waters is desirable not only for the diluting and concentrating solutions but also as the solutions fed to the washing and electrode chambers. It will be obvious, therefore, that precipitates can be formed in the cathode chamber by the products of the electrolysis formed therein. Such precipitates are to a large extent flushed out of the cathode chamber if a sufficiently high velocity of the flowing solution is maintained. It will be obvious, however, that for continued operation, the retention of even a small percentage of the precipitate in the cathode chamber wil eventually result in the obstruction of the free flow of solution through the chamber with the evolution of gaseous electrolysis products, and also prevent uniform passage of electrical current through the chambers. Also it will be apparent that economical operation of the demineralizer will not permit the use of large volumes of electrode solution. I, therefore, provide the electrode chambers of our demineralizer with spacers of similar tortuosity to the spacers noted above (although perhaps of considerably greater thickness) and direct the flow of the electrode solutions in parallel to each other and to the flow of solutions in the concentrating and diluting chambers at a rate sufficient to result in a hydraulic pressure similar to that effected in the other chambers. The

efliuents from the electrode chambers are collected in a common reservoir which combined effluent is then recycled to the electrode chambers, after the electrode gases are disengaged. I simultaneously bleed in required quantities of feed solution and permit the overflow of a similar volume of waste from the electrode reservoir.

To assure that no precipitation will accumulate in the cathode chambers, I bleed in quantities of acid suflicient to insure that the solution leaving the cathode chamber I will have a pH of less than 5 and preferably not less than 1. A suitable acid maybe HCl acid although sulfuric acid and acid sodium sulfate may be used if the feed solution to the recycling electrode stream does not contain such a high concentration of calcium that the introduction of sulfate ion would cause precipitation of CaSO In the demineralization of naturally occurring waters by the present invention, it is economical and entirely feasible that the feed streams for all three diluting, concentrating, and electrode circulations be composed of the original naturally occurring waters. However, in the treatment of industrial solutions the concentrating and electrode feed streams for obvious reasons should not be the industrial solution to be demineralized. For example, in the demineralization of a sugar and glycerine solution an appropriate feed to the concentrate stream is a tap water, and the feed to the electrode stream could be a dilute sodium acid sulfate solution or sodium sulfate solution.

This invention will be more fully understood from the following detailed description of representative embodiments of the invention selected for purposes of illustration wherein reference is made to the drawings in which:

Figure 1 is a schematical representation of the complete system which includes a concentrating and diluting cell unit and the circulatory system in connection therewith.

Figure 2 is a perspective view of the elements of a pair of concentrating and diluting chambers in exploded relationship.

A concentrating and diluting cell unit U shown schematically in Figure 1 consists, in general, of a series of parallel alternating diluting and concentrating chambers, and 13 respectively, divided by alternating membranes selectively permeable to cations K and anions A, defining end electrode chambers S and 9a with cathode and anode electrodes 7 and 5 attached thereto on one side thereof and a pair of washing chambers 11 adjacent thereto on the other side. A direct current for passage through the cell is obtained through leads 1 and 3 from a suit-able source (not shown). The membranes A and K are electrically conductive, as well as selectively permeable to anions and cations, respectively. Suitable membranes for this purpose are described in the copending application to W. E. Katz and Norman W. Rosenberg noted above. The washing and electrode chambers do not function directly in the concentrating and diluting process, but are provided to minimize contamination of the solutions in the concentrating and diluting chambers by the products of electrolvsis. The washing chambers maybe omitted if desired.

In Figure l, the concentrating, diluting, washing, and end electrode chambers are fed in parallel with the washing chambers being fed and removed in the concentrate stream. The washing chambers could also be hydraulically independent if desired. The anode and cathode streams are fed separately in parallel and removed in a single stream. The anode and cathode streams could also be removed separately from the unit U into separate electrode reservoir tanks if desired.

In the operation of the cell unit U separate streams of feed solution to be diluted and concentrated are passed through the diluting chambers and concentrating chamhere in parallel concurrent flow. An electrolytically conductive solution is also passed in parallel through anode and cathode electrode chambers and out in a common stream. A direct electric current is then passed through the cell which causes cations to cross the cation selectively permeable membrances K in migration towards the cathode, and anions to cross the anion selectively permeable membranes A in migration towards the anode, there by, each diluting chamber becomes depleted in its electrolyte content, while the alternating concentrating chambers receive this electrolyte. Current is conducted to the battery of concentrating and diluting chambers through the solutions in the electrode chambers with negligible contamination of the solution being treated by the products formed at the electrodes.

The circulatory systems for the three streams employed in the system of the disclosed embodiment of the invention are designated broadly by D (diluting stream), C (concentrating), and E (electrode stream). These are separate streams which originate from their sources 2, 50 and 100, into feed'tanks 4, 52 and 102, respectively. They enter and leave the electrodialysis cell unit U through conduits 38 and 40; 80 and 82; and 128 and 130, respectively.

The diluting stream D originates from a source 2 supplying feed tank 4, is pumped by centrifugal pump 6 through filter 10 which has pressure gages 8 and 12 on both sides thereof to indicate the degree of exhaustion of said filter. Feed valev 14 is normally kept in a closed position to prevent the raw feed stream from entering the recirculating diluting system but opened periodically to fill reservoir tank 18. This valve may be,

for example, of the solenoid, pneumatic, or motor-driven type. The batch of solution to be diluted in reservoir diluting tank 18 is pumped through conduit 44, by centrifugal pump 30, throttled through valve 32, and passed through flowrator 34. Its pressure is measured on pressure gage 36, and it then flows through main conduit'38 to the diluting chambers of the electrodialysis cell unit through main conduit 40, passes through conductance measuring cell 28 and normally open valve 20 (valve 22 being normally closed), returning to the diluting reservoir tank 18 for recirculation. cell 28 indicates satisfactory dilution has been achieved, valve 20 is closed and normally closed valve 22 is opened. The product steam leaving the unit is thus by-passed through conduit 46 into product tank 24 by means of pump 22, and diluting reservoir tank 18 empties through outlet 26 therefrom. When tank 18 is almost empty,

' a float switch therein (not shown) opens valve 14, allowing a fresh batch of raw solution to be diluted to enter the stream by pump 30. Because of its initial high con- When the conductance ductivity when the raw solution to be diluted enters conductance cell 28, valve 22 is normally closed and valve 20 is normally open. Diluting tank 18 then refills the raw solution to be diluted and when full, valve 11 is closed, cutting oif'further entry of raw solution to be diluted. The unit continues to demineralize this new batch in tank 18 until it is sufficiently pure agains initiating the cycle, This method of operation is. referred to hereinafter as batch circulation.

An alternative method of operating the diluting circuit is to adjust valve 14 to a partially open position thereby continuously feeding solution to be diluted through flowrator 16 and continuously withdrawing product at exit overflow 19 in which case valve 20 is continuously open, and valve 22, conduit 46, and product tank 24 are eliminated. This method of operation is referred to as continuous circulation and removal of the diluting stream as opposed to the batch recirculation described in the preceding description, Note especially that the recirculation rate through fiowrator 34 and the feed rate through flowrator 16 are completely independent of one another and may be varied relative to one another.

The concentrating stream C originates from conduit 50 into concentrating stream feed tank 52 and is then pumped through centrifugal pump 54, through filter 58 which has pressure gages 56 and 60 on both sides thereof for determining the extent of exhaustion thereof. The concentrating stream is then passed through partially open valve 62 and flowrator 64 which measures the feed rate of the concentrate feed stream. It is joined by the main recirculating concentrate stream which is brought from concentrate recirculating tank 68 through conduit 66 by virtue of centrifugal pump 72, through throttling valve 74, through fiowrator 76, its pressure being measured on pressure gage 78, enters the electrodialysis unit through conduit 80, is removed from the electrodialysis unit U through conduit 82, passed through conductance cell 70 and thence returns to the concentrating recirculating reservoir tank 68. A continuous overflow from reservoir 68 through conduit 86 is provided. This concentrate stream could also be fed and removed in a batch-like manner identical to that used on the diluting stream and described as batch recirculation above. In such a case, the conductance cell 70 would indicate the highest permissible concentration and would signal the opening of valve 62. If this concentration were exceeded, the opening of valve 62 would introduce a quantity of solution to be concentrated at a lower concentration than that being removed through conduit 86. It is clear that these various instruments may either function automatically or be manually controlled.

A solution appropriate for use in the electrode stream, e. g. sodium sulfate or any natural water such as sea water, is introduced through conduit 100 into electrode feed tank 102. From this tank it is pumped through centrifugal pump 104, through filter 108 which has pressure gages 106 and 110 at each end to indicate the degree of exhaustion of the filter, is then passed through partially open throttle valve 112, and through flowrator 114 where it is joined by the main recirculating electrode stream from electrode reservoir tank 118 through conduit 116. The combined stream is then passed through centrifugal pump 120, throttle valve 122, fiowrator 124, its pressures measured on gage 126, and introduced into the electrodialysis unit U through main conduit 128, removed therefrom through return conduit 130 at which point it is returned to the electrode stream recirculating reservoir 118. A continuous overflow from the electrode reservoir tank 118 is provided through conduit 132. If desired, a small stream of acid may be added to electrode reservoir 118 from acid container 134 through throttle valve 136, and introduced into the recirculating stream through conduit 138 to maintain a pH of the electrode solution at such a value that precipitation will not occur in the electrode compartments. It is obvious that batch operation of this electrode stream could also be untilized.

In the case where a valuable solution is to be concentrated, it may be permissible to utilize a single source of feed solution for both concentrate and electrode recirculating streams by connecting the conduit leading from filter 58 through broken line conduit 84 to feed valve 112, as well as to valve 62, permitting the elimination of the separate electrode feed system indicated by components 100, 102, 104, 106, 108, 110. In the case of a vaiuable strzarn to be diluted, one might similarly connect a single source of feed solution to the diluting and electrode conduit feed valves. In the case of a natural Water, a single feed system might well be used for all three recirculating streams by connecting broken line conduits 42 and 84 as shown in the diagram in Figure l and eliminating componentsSO, 52, 54, 56, 58, and 60 and components 100, 102, 104, 106, 108, and 110.

Figure 1 shows three separate streams 38, 80, and 128, feed to electrodialysis unitU and threeseparate streams 40, 82 and 130 leaving the unit. Alternating individual chambers 13 and 15 are shown bounded by cation and anion permeable membranes K and A wherein entrys are affected at points 25 and 29 and the solution leaving individual chambers at points 27 and 31 respectively (see Figure 2). In general, it will not be desirable to have the rate of flow through conduit equal the rate of flow through conduit 38 and therefore the pressure drop encountered between 25 and 27 will not necessarily equal the pressure drop occurring between 29 and 31. In this general situation, therefore, pressure will be exerted across the face of the membranes from chamber to chamber and deformation and leakage described above can become serious especially when the pressure drops from 25 to 27 or 29 to 31 become very high. To correct this situation, recirculation pumps 30 and 72 are throttled by valves 32 and 74, respectively, in such a manner that pressure gages 36 and 78 have the same pressure reading which indicates, if pressure drop through manifold pipe are small, that points 25 and 29 are at the same pressure. Points. 27 and 31 are similarly maintained at an equal atmosphere pressure and hence the membranes in the chambers of the unit at any point thereof will be approximately the same and will not accordingly be exposed to deformation forces. Similar consideration requires that the electrode stream entering through conduit 128 similarly be throttled by valve 122 until the pressure indicated on gage 126 is equal to that on pressure gages 36 and 78 which then assures that the membranes at the entries of the unit will not be exposed to any significant deformation forces. The recirculation rates in the system described in Figure 1 can be chosen completely independently of the desired feed rates to the dilute stream, concentratestream, and the electrode stream, which are determined respectively by the setting of valves 14, 62, and 112 or by the set points indicated on conductance cells 28 and 70. In other words, recirculation rates are chosen to maintain balanced forces across all membranes whereas feed rates are chosen in accordance with the requirements of the streams to be processed.

Figure 2 shows an exploded perspective view of a pair of concentrating and diluting chambers of the electrodialysis cell unit (U) of Figure 1, like numerals for like parts being employed. Numerals 21 and 23 indicate one form of baffle employed to effect a tortuous path for the flow streams in the concentrating and diluting chambers. It is apparent that the multiple chambers could very well be expanded into several hundred of such chambers.

The embodiment of the recirculation system of Figure 1 shows the diluting stream to be batch recirculation while both the concentrating and electrode streams are continuous recirculation systems, but it should be appreciated that many varied combinations of the diluting and concentrating streams in conjunction with the electrode stream is contemplated herein as noted above.

amass The following table lists some of the most desirable combinations of flow streams:

Dilute Concen- Elec- Stream tration trode Stream Stream H C C b c c n b c b b c c c c c b c where,

The following examples are given to further show the operation of the system with various feed streams and the results obtained.

EXAMPLE 1SEA WATER In a unit constructed as shown in Figure 1 (with broken line conduits 42 and 84 in operation), the following performance was obtained using an 80 cell pair demineralizer stack with .10 cm. thick spacers except at electrodes which were .32 cm. thick. Raw sea water at 40 F. was contained in tank 4, which had a capacity of 500 gallons. Valve 14 is normally in the closed position. The diluting stream was recirculated from the ten-gallon reservoir tank 18 through the electrodialysis unit (U) and by means of valve 32 was adjusted to a flow of 300 G. P. H. measured on flowrator 34 at a pressure read on gage 36 of 12 p. s. i., returning to the diluting reservoir tank 18. Recycle of the concentrating stream from S-gallon tank 68 was at a 300 G. P. H. flow and 12 p. s. i. pressure, equal to the pressure on the dilute recirculating stream. Raw sea water fed to the concentrate stream and measured through fiow rator 64 was adjusted by valve 62 to 20 G. P. H. when the pressure read on gage 12 was 15 p. s. i. Recycle of the electrode stream was 25 G. P. H. through flowrator 124 at a pressure reading on gage 126 of 12 p. s. i., equal to the pressure on the dilute recirculating stream. Raw

water feed to electrode stream measured on flowrator 114 was 4 G. P. H. when the pressure read on gage 12 was 15 p. s. i. Sulfuric acid at a concentration of 25% was fed at a rate of 0.1 G. P. H. from container 134, creating an overflow pH of 2.3 during normal unit operation from tank 118.

A cycle lasted for 35 minutes when 65 volts D. C. was impressed across the electrodes of the cell unit. The initial current was 20 amperes. As the salt level of the water in tank 18 fell to 500 p. p. m. the current had decreased gradually to 3 amperes. At this level the electrical circuit controlled by the conductance cell 28 caused valve 20 to close and valve 22 to open, allowing tank 18 to empty into tank 24. When the level in tank 18 fell to 1 gallon, the electrical circuit controlled by the float switch therein (not shown) caused valve 14 to open, and a stream of raw water entered tank 18. As the raw water passed through the unit, it forced the purified water ahead of it from the unit into product tank 24. As soon as a mixture of product and raw water of above 500 p. p. m. salt content passed through conductance cell 28,- v

valve 20 opened and valve 22 closed, and when tank 18 filled to the 10-gallon level, the float switch closed valve 14, allowing a new cycle to begin automatically.

After 10 days of continuous uninterrupted operation,

the unit described above produced 15 gallons per hour of 4 500 p. p. m. demineralized water at a D. C. power con sumption of 1 kilowatt.

When disassembled, there was no sign of precipitate adhering to the electrode chambers or any of the diluting or concentrating chambers.

EXAMPLE 2BRACKISH WATER In the same unit used in Example 1, the tank 4 was filled with a brackish water containing 5000 p. p. m. of

salt. No other adjustments were made; that is, all feed and recirculating lines operated at the same conditionsfeed rates of concentrating and electrode streams remained unchanged. The only changes were that the unit, with 65 volts D. C. across electrodes, passed an initial current of 10 amperes and a final current of 2.5 amperes, and the cycle time for the batch was much shorter, only 12 minutes in length. Thus 45 gallons per hour were produced at a power consumption of 500 watts.

After a period of three days of continuous and uninterrupted operation the unit was disassembled and no sign of precipitate was apparent.

EXAMPLE 3--INDUSTRIAL SOLUTIONS Cane juice of 150 G. P. H. at a pressure of 10 p. s. i. and at a temperature of 60 C. Tap water was fed to the concentrate stream which stream was recycled at a flow of 300 G. P. H. to obtain a pressure of 10 p. s. i. Recycle of a .1 N NaHSO stream was 25 G. P. H. through anode and through cathode chambers at a pressure of 10 p. s. i. Reduction of the conductivity of the raw sugar to 10% of its original value was accomplished at 65 volts D. C. in a period of 240 minutes when the batch volume at the start of the run was 10 gallons. The starting current was 5 amperes and the final current 1.2 amperes, thus indicating a production rate of 2.5 gallons per hour at an average power demand of 0.2 kw., with the ionizable ash content reduced as measured by conductance. As is well known in the art of sugar production, the presence of ionizable ash prevents crystallization of sucrose from the cane juice to the extent of about 1 pound of sugar retained per pound of such ash. Thus it is economically advantageous to remove this ash in order to produce more granulated sugar and less molasses, which molasses is the residue after crystallization of all possible sugar and has a lower economic value than the granulated sugar.

EXAMPLE 4--INDUSTRIAL SOLUTIONS Glycerine The unit described in Example 3 was operated in an identical manner on a glycerine process stream containing 30% glycerine and 3% of ionizable solids, largely NaCl. The usefulness of removing the ash from glycerine is that only water need be removed to produce a glycerine suitable for industrial use. If, on the other hand, the salt is not removed the glycerine must be distilled away from the salt to produce a commercially useful product, the latter being an expensive process. Removal of the entire salt content by the usual granular ion exchange resin beds is not feasible because of the large quantity of salt present. It was found that a recirculating flow of G. P. H. was obtained in the diluting stream at 15 p. s. i. pressure, and that a 350 G. P. H. stream of tap water was necessary in the concentrate stream to provide the same pressure. The electrode streams required a low of 30 G. P. H. through anode and 30 G. P. H. through cathode chambers to maintain this same pressure. When a pressure of 5 p. s. i. on the concentratingstrea n was utilized, significant glycerine leak: age from the stream to be treated into the concentrate reservoir was obtained presenting a problem in recovery of this glycerine from the concentrating stream. However,

at the pressures utilized, which were equal and balanced,

this leakage was significantly reduced, therebydecreasing the recovery problem. Inthe above unit a voltage of 100 volts;impressed.between electrodes resulted-sin an initial current of 12 amperes and a current of '2iamperes,

when; the solution showed a 95% conductancedecrease. A cycle timeof .1 hour was required for-an initial batch of 12 gallons of process stream solution, andthus a D C..

power of .9 kw. and a production rate of 12G. P. H was obtained from this unit in the treatment of this solution.-

EXAMPLE 5INDUSTRIAL SOLUTIONS" Whey A system as shown in Example 3 containing 10 cncentrating chambers and 9 diluting chambers was utilized to remove ash and lactic acid from a whey sample. A

2-liter sample of clarified whey was placed in a flask and stirred. A stream of 16 ml. per minute was passed from the diluting reservoir tank 18 through the electrodialysis unit (U) and back to the reservoir tank 18. A stream of .2 N NaCl was passed through theconcentrating stream from reservoir tank 68, and a stream of .2 N H 80 was passed through the electrode stream at approximately equal flow rates. showed thata pressure difference as low as, two inches of water caused a 5% cross-leak but that at balanced pressures the cross-leak was negligible. When operating at such balanced pressures, initially 6.5 volts was sufficient to pass 150 milliamperes through the solution which solution had a resistance of 83 ohms corresponding to the concentration of 1.25 N in ionized solids. At the end of five hours this concentration had fallen to .060 N and 9.7

volts was required to pass 75 milliamperes. At the end of twelve hours the concentration was reduced further and 9.3 volts was required to pass 16.5 milliamperes. Chemical analysis indicated that the pH was reduced by treatment from 4.14 to 3.66. Total solids decreased from 4.25 to 3.55. Ash reduced from 1.55 to .002, protein reduced .20 to .08, and lactic acid reduced from. 2.8 to 0.2 by the above treatment. All numbers expressed as percentages of solution.

It is understood that the valves, pumps, flowrators, conductance cells, pressure gages, and safety devices including pressure points, timing switches, etc., employed in the system are well known, per se, but the assembly and incorporation of the same in conjunction with the novel present system wherein sensing and automatically controlling the operation of membrane demineralizing of liquids may be obtained are novel and may be valuable features to the commercial success of the present invention.

Having thus disclosed my invention and described in detail representative and preferred embodiments thereof, I claim and desire to secure by Letters Patent:

1. The method of modifying the concentration-of an electrolyte solution comprising passing a first feed stream through diluting chambers of an electrodialysis unit having a plurality of concentrating and diluting chambers defined between alternate anion permeable membranes and cation permeable membranes said membranes having a thickness between about 0.1 mm. and about 1 mm. and.

said diluting chambers having a thickness of about-0.5 mm. to about 2 mm, passing a second feed stream through the alternate concentrating chambers, passing a direct current in series across the alternating chambers and membranes, and passing the two feed streams in parallel concurrent flow through said chambers at flow rates controlled to maintain substantially equal hydraulic pressures on opposite faces of the membranes at any point thereof to prevent undue stresses or bowing of said membranes.

The first experiments 2. The methodof modifying the concentration; of an electrolyte solution comprising ,passing a first feed stream through diluting chambers of an electrodialysis unit having a plurality of concentrating and diluting chambers defined between alternate anion permeable membranes and cation permeable membranes, passing a second feed stream through the alternate concentrating chambers, passing a direct current in series across the alternating chambers and membranes said membranes having a thickness between about 0.1 mm. andabout 1 mm. and said diluting chambers having a thickness of about 0.5 mm. to about 2 mm., passing the two feed streams in parallel concurrent flow through said chambers, and at least partially recirculating at least one of said feed streams at a fiow rate to. maintain.substantially equal hydraulic pressures on opposite faces ofthe membranes at any point thereof to prevent undue stresses or bowing .of said membranes.

3. The method .of modifying the concentration .of .an electrolytesolution comprising passing a first feed stream through diluting chambers of an electrodialysis unit having a plurality of concentrating and diluting chambers defined between alternate anion permeable membranes and cation permeable membranes, said membranes having a thickness between about 0.1 mm. and about 1 mm. and said diluting chambers having a thickness of about 0.5 mm. to about 2 mm., and end electrode chambers, passing a second feedstream through the alternate concentratingchambers, passing a third stream through the end electrodechambers of the unit, passing a direct current in series across the alternating chambers and membranes, passing the feed streams in parallel concurrent fiow through their respective chambers and recirculating at least part of at least one of said streams at a fiow rate to maintain substantially equal hydraulic pressures on opposite faces of the membranes at any point thereof to prevent undue stresses or bowing of said membranes.

4. The method of modifying the concentration of an electrolyte solution comprising passing a first feed stream through diluting chambers of an electrodialysis unit having a plurality of concentrating and diluting chambers definedbetween alternate anion permeable membranes and cation permeable membranes, said membranes having a thickness between about 0.1 mm. and about 1 mm. and said.diluting chambers having a thickness of about 0.5 mm. to about 2 mm., and end electrode chambers, passing a second feed stream through the alternate concentrating chambers, passing a third feed stream through the end electrode chambers of the unit, passing a direct current in series across the alternating chambers and membranes, passing the feed streams in parallel concurrentflow through their respective chambers and recirculating at least part of all said streams at a flow rate to maintain substantial equal hydraulic pressures on opposite faces of the membranes at any point thereof to prevent-unduestresses or bowing of said membranes.

5. The method of claim 2 wherein the recirculating stream is the dilute stream and consists of continuously recirculatingv all ,of the stream and periodically removing and replacing at least part of said stream with raw feed solution.

6. Themethod of claim 2 wherein the recirculating stream is the dilute stream and consists of partially recirculatingsaid stream and continuously, removing and replacing ,at least partof said stream with raw feed solution.

7. The method of claim 4 wherein at least part of the diluting stream is periodically removed and replaced with rawfeed solution.

8. The method of claim 4 wherein the dilute stream is partially recirculated and continuously removed and replaced with raw feed solution.

9. The method of claim 4 wherein sufficientacid is addedto the electrodefeed recirculating stream to maintainanacidpI-I in said'stream thereby preventing metal 13 i4 salt precipitation in the cathode electrode chamber of 504,756 Belgium Aug. 14, 1951 the unit. 682,703 Great Britain Nov. 12, 1952 694,223 Great Britain July 15, 1953 References Cited in the file of this patent 67,903 Netherlands May 17, 1951 U I T P N STA ES ATENTS 0 OTHER REFERENCES 1,868,955 Taclnkawa July 26, 1932 2,132,391 Skolnik 5 1939 Arnoerplex Ion Permeasle Membranes, Rohm and 2 3 5 457 Daniel Dec. 19 1944 Haas 00., Philadelphia, P21. 95 PP- 9, 2 and 3- 2,411,239 Reichel et Nov. 19 1946 Langelier, Journal of the American Water Works As- 257L247 Huebottel- Oct 6) 1951 10 sedation, p 1952, PP 845 to 2,683,117 Rosenak et al July 6, 1954 FOREIGN PATENTS 211,562 Great Britain Feb. 20, 1924 UNITED STATES PATENT OFFICE Certificate Patent No 2,848,403 Patented August 19, 1958 Norman W. Rosenberg Application having been made jointly by Norman W. Rosenberg, the inventor named in the patent above identified; Ionics, Incorporated, Cambridge, Massachusetts, a corporation of Massachusetts, the assignee; and Wayne A. McRae of Lexington, Massachusetts, for the issuance of a certificate under the provisions of Title 35, Section 256 of the United States Code, adding the name of the said Wayne A. McRae to the patent as a joint inventor, and a showing and proof of facts satisfying the requirements of the said section having been submitted, it is this 2nd d y of February 1965, certified that the name of the said Wayne A. McRae is hereby added to the said patent as a joint inventor With the said Norman W. Rosenberg.

[SEAL] EDWIN L. REYNOLDSH: First Assistant Commissioner 0 Patents. 

1. THE METHOD OF MODIFYING THE CONCENTRATION OF AN ELECTROLYTE SOLUTION COMPRISING PASSING A FIRST FEED STREAM THROUGH DILUTING CHAMBERS OF AN ELECTRODIALYSIS UNIT HAVING A PLURALITY OF CONCENTRATING AND DILUTING CHAMBERS DEFINED BETWEEN ALTERNATE ANION PERMEABLE MEMBRANES AND CATION PERMEABLE MEMBRANES SAID MEMBRANES HAVING A THICKNESS BETWEEN ABOUT 0.1 MM. AND ABOUT 1 MM. AND SAID DILUTING CHAMBERS HAVING A THICKNESS OF ABOUT 0.5 MM. TO ABOUT 2 MM. PASSING A SECOND FEED STREAM THROUGH THE ALTERNATE CONCENTRATING CHAMBERS, PASSING A DIRECT CURRENT IN SERIES ACROSS THE ALTERNATING CHAMBERS AND MEMBRANES, AND PASSING THE TWO FEED STREAMS IN 